1. Field of the Invention
This invention relates generally to a powertrain for a motor vehicle, and, more particularly, to a powertrain having multiple power sources including an electric motor for driving a set of vehicle wheels.
2. Description of the Prior Art
In a powertrain for a hybrid electric vehicle (HEV), inertial masses and drag losses offset the fuel economy, performance, and dynamic response gains of the hybrid system. These offsets are greater when the hybrid drive components are not substituted for standard powertrain components.
Electric rear axle drive units for front wheel drive vehicles add additional components and thus inertias and drag losses in both electric all wheel drive and shaft-driven mechanical all wheel drive systems. Electric front axle or rear axle drives for rear wheel drive vehicles added on top of the existing longitudinal driveline present the same problems.
The inertial masses include the added motor/generator rotor, gear assemblies, and shaft and hub assemblies. The drag losses include the additional gear and bearing losses of the drive components and electromagnetic drag losses in motor assemblies. The various drag losses can be reduced by design detail, but cannot be eliminated.
These inertias and drag torques are further multiplied by the gear ratios often present between the motor/generator and their mechanical outputs to the drivetrain.
It would be desirable to reduce the effects of inertial masses and drag torques, especially when the electric drive is not in operation. A need exists for a technique to connect and disconnect an electric motor for a front axle drive or rear axle drive motor from the driveline or the respective axle so that inertia and drag losses can be reduced. Controlled hydraulically-actuated friction clutches for this purpose increase the complexity of a hydraulic system and fluidic drag losses.